The present invention relates generally to the in situ formation of hydrogels, and more specifically, to compositions of hydrogels that are formed in situ by a combination of physical and chemical crosslinking.
Hydrogels are materials which absorb solvents (such as water), undergo rapid swelling without discernible dissolution, and maintain three-dimensional networks capable of reversible deformation. Hydrogels have been proposed for use in a number of medical applications, including barriers to prevent post-surgical adhesion, as tissue adhesives, as bone implants, and for occluding naturally-occurring and iatrogenically formed lumens.
Hydrogels may be either uncrosslinked or crosslinked. Crosslink formation in polymers is usually accompanied by an increase in viscosity due to an increase in apparent or real molecular weight, and often may result in the formation of a gelled state.
Polymers may be crosslinked by either physical or chemical means. Physical crosslinking differs from chemical crosslinking in that the linkages are typically weaker, of lower energy, and often reversible. Thus, physically crosslinked hydrogels often are deformable mechanically. Four fundamental forces have been found to be responsible for producing physical crosslinking: ionic interactions; hydrophobic interactions; hydrogen bonding and Van der Waals forces.
Gel forming compositions for use in preventing post-surgical adhesion are known. For example, U.S. Pat. No. 4,994,277 to Higham et al. describes a barrier to prevent post-surgical adhesions formed from a water soluble xanthan gum gel. The water solubility of the gel may lead to inadequate retention duration, and risk of displacement and migration.
U.S. Pat. No. 4,911,926 to Henry et al. describes methods and composition for reducing post-surgical adhesions by using aqueous and non-aqueous compositions comprising polyoxyalkylene block copolymers that form gels in the biologic environment.
The hydrogels described in the foregoing patent form weak physical crosslinks that give the gels a paste-like consistency at physiological temperatures. Because the resulting gels are not covalently crosslinked, however, they have no mechanical integrity and may be readily displaced from the application site.
U.S. Pat. No. 5,126,141 to Henry describes compositions and methods for reducing post-surgical adhesions with thermo-irreversible gels of polyoxyalkylene polymers and ionic polysaccharides. Both the polyoxyalkylene and ionic polysaccharide are crosslinked by physical crosslinks; no covalent crosslinking is disclosed.
Other gel forming compositions for use in preventing post-surgical adhesion have included: (a) chitin derivatives (U.S. Pat. No. 5,093,319) (b) chitosan-coagulum (U.S. Pat. No. 4,532,134); and (c) hyaluronic acid (U.S. Pat. No. 4,141,973). There is no clear mechanism for degrading these tonically crosslinked materials, or their constituent molecules, which may remain within the body for uncertain periods of time.
Likewise, U.S. Pat. No. 5,266,326 to Barry et al. describes in situ modification of alginate to form a hydrogel spinal implant in vivo. Ionically crosslinked polysaccharides such as alginate are not absorbable in humans since no enzyme exists in humans to degrade the xcex2 glycosidic linkages. Also, the high molecular weight of the alginates used (upwards of 200,000 Da) does not allow filtration through the kidneys.
Covalently crosslinked hydrogels have been prepared based on crosslinked polymeric chains of methoxy poly(ethylene glycol) monomethacrylate having variable lengths of the polyoxyethylene side chains. The interaction of these hydrogels with blood components is reported in Nagaoka, et al., in Polymers as Biomaterial (Shalaby et al., Eds.) Plenum Press, 1983, p. 381.
U.S. Pat. No. 5,573,934 to Hubbell et al. describes ethylenically unsaturated water soluble macromers that can be crosslinked in contact with tissues, cells, and bioactive molecules to form gels. The patent does not describe the inclusion of physically crosslinkable components to facilitate in situ formation of hydrogels.
U.S. Pat. No. 4,740,534 to Matsuda et al. describes surgical adhesives comprising urethane prepolymers used in combination with an unsaturated cyano compound containing a cyano group attached to a carbon atom, such as cyano(meth)acrylic acids and esters thereof.
U.S. Pat. No. 4,804,691 to English et al. describes a method of making an adhesive that is polymerized in situ to join soft living tissue, including steps of preparing a hydroxyl-terminated polyester by reacting a biodegradable monomer with a polyhydroxy polymerization initiator. When applied to the moist soft tissue, the adhesive reacts with water to yield a cured adhesive having a resorbable backbone with urethane linkages. The adhesive is not water soluble, however, and thus cannot intimately mix with tissue fluids. Also, because the adhesive reacts with water, it cannot be applied as an aqueous solution or suspension, and has poor adherence in the presence of excess surface moisture on tissue.
U.S. Pat. No. 5,462,976 to Matsuda et al. describes in situ polymerizable tissue barriers formed from photocurable glycosaminoglycan derivatives that are water soluble in their precursor form. These materials require external energy sources for transformation and polymerize slowly. U.S. Pat. No. 5,410,016 to Hubbell et al. describes biodegradable hydrogels that polymerize more rapidly by a photoinitiated free radical polymerization from water soluble macromers.
Synthesis and biomedical and pharmaceutical applications of absorbable or biodegradable hydrogels based on covalently crosslinked networks comprising polypeptide or polyester components as the enzymatically or hydrolytically labile components, respectively, have been described by a number of researchers. See, e.g., Jarrett, et al., xe2x80x9cBioabsorbable Hydrogel Tissue Barrier: In Situ Gelation Kineticsxe2x80x9d, Trans. Soc. Biomater., Vol. XVIII, 182 (1995); Sawhney et al., xe2x80x9cBioerodible Hydrogels Based on Photopolymerized Poly(ethyleneglycol)-copoly(xcex1-hydroxy acid) Diacrylate Macromersxe2x80x9d, Macromolecules, 26:581-587 (1993); Park, et al., Biodegradable Hydrogels for Drua Delivery, Technomic Pub., Lancaster, Pa., 1993; Park, xe2x80x9cEnzyme-digestible Swelling Hydrogels as Platforms for Long-term Oral Drug Delivery: Synthesis and Characterizationxe2x80x9d, Biomaterials, 9:435 (1988). None of these references suggest the desirability of providing physical crosslinking in addition to chemical crosslinking to improve hydrogel formation in situ.
Several previously known chemical systems are compatible with carrying out chemical reactions or crosslinking in vivo to form covalently crosslinked hydrogels. Monomers or macromers usable to form hydrogels by chemical crosslinking may be viewed as comprising two broad categories, depending on the kind of functional groups that are active in the crosslinking process. When these functional groups are self condensible, as is the case for example for ethylenically unsaturated functional groups, the crosslinker is by itself sufficient to result in the formation of a hydrogel, when polymerization is initiated with appropriate agents. In situ polymerized hydrogels also can be formed by crosslinking reactions that are a result of a reaction between dissimilar functional groups that have a tendency to react under physiological conditions in timeframes relevant for in situ polymerization.
U.S. Pat. No. 5,122,614 to Zalipsky describes that PEG molecules activated with an oxycarbonyl-N-dicarboximide functional group can be attached under aqueous, basic conditions by a urethane linkage to the amine group of a polypeptide. Activated PEG-N-succinimide carbonate is said to form stable, hydrolysis-resistant urethane linkages with amine groups. The amine group is shown to be more reactive at basic pHs of from about 8.0 to 9.5, and reactivity falls off sharply at lower pH.
The foregoing patent to Zaplinsky also describes several other previously known PEG derivatives. PEG-succinoyl-N-hydroxysuccinimide ester is said to form ester linkages having limited stability in aqueous media, thus indicating an undesirable short half-life. PEG-cyanuric chloride is said to exhibit an undesirable toxicity and to be non-specific for reaction with particular functional groups on a protein. PEG-phenylcarbonate is said to produce toxic hydrophobic phenol residues that have affinity for proteins. PEG activated with carbonyldiimidazole is said to be too slow in reacting with protein functional groups, requiring long reaction times to obtain sufficient modification of the protein.
Still other PEG derivatives have been proposed for attachment to functional groups on amino acids other than the epsilon xe2x80x94NH2 of lysine. Histidine contains a reactive imino moiety, represented structurally as xe2x80x94N(xe2x80x94H)xe2x80x94, but many derivatives that react with epsilon xe2x80x94NH2 also react with xe2x80x94N(xe2x80x94H)xe2x80x94. Cysteine contains a reactive thiol moiety, represented structurally as xe2x80x94SH, but the PEG derivative maleimide that is reactive with this moiety is subject to hydrolysis.
Considerable effort regarding previously known compositions has gone into developing various PEG derivatives for attachment to, in particular, the xe2x80x94NH2 moiety on the lysine amino acid fraction of various proteins. Many of these derivatives have proven problematic in their synthesis and use. Some form unstable linkages with the protein that are subject to hydrolysis and therefore do not last very long in aqueous environments, such as in the blood stream. Some form more stable linkages, but are subject to hydrolysis before the linkage is formed, which means that the reactive group on the PEG derivative may be inactivated before the protein is attached. Some are somewhat toxic and are therefore less suitable for use in vivo. Some are too slow to react to be practically useful. Some result in a loss of protein activity by attaching to sites responsible for the protein""s activity. Some are not specific in the sites to which they will attach, which can also result in a loss of desirable activity and in a lack of reproducibility of results. However, in all of the foregoing cases, the object of the modification was to form a soluble product, not a crosslinked hydrogel.
U.S. Pat. No. 5,583,114 to Barrows et al. describes an adhesive composition formed from a two component mixture including a first part comprising a serum albumin protein in an aqueous buffer having a pH in a range of about 8.0-11.0, and a second part comprising a water-compatible or water-soluble bifunctional crosslinking agent. Several minutes are needed for these materials to polymerize and only serum albumin is appropriate for reacting with bifunctional crosslinking agents. The crosslinking agents described in the Barrows patent are only PEG based and are also relatively slow acting.
International Publication WO 97/22371 to Rhee et al. describes the use of a hydrogel composition that is produced by the reaction of a synthetic nucleophilic polymer containing two or more nucleophilic groups with a second synthetic polymer containing two or more electrophilic groups. The compositions described in the patent take from 5 to 50 minutes to achieve crosslinking. Moreover, electrophilic functional groups that are expected to be more reactive are also reactive with the hydroxyl groups of water and thus have very limited stability at physiological pHs. Accordingly, the compositions described in that reference are unsuitable for tissue sealing and/or coating type applications, because it would be difficult to retain the hydrogels at the application site for the extended periods of time required for reactions to take place with shelf stable compositions.
In view of the foregoing, it would be desirable to provide methods and compositions for forming a hydrogel implant in situ by a combination of physical and chemical crosslinking of solutions of substantially water soluble precursors, wherein the physical crosslinking assists in retaining the hydrogel in place at a deposition site while the more durable chemical crosslinks are formed.
It further would be desirable to provide methods and compositions that result in the formation of absorbable or non-absorbable hydrogels by appropriate structural design of the hydrogel forming precursors.
It still further would be desirable to provide physically and chemically crosslinked hydrogels systems suitable for use in medical applications, such as reducing post-surgical tissue adhesion, plugging lumens, sealing tissue, gluing tissues together, and supporting or augmenting tissue either from within the tissue matrix, over the tissue surface, or as an adjunct with another supporting biomaterial.
It also would be desirable to provide methods and compositions for controlled and local, regional or systemic drug delivery.
In view of the foregoing, it is an object of the present invention to provide methods and compositions for forming a hydrogel implant in situ by a combination of physical and chemical crosslinking of solutions of substantially water soluble precursors.
It is also an object of the present invention to provide methods and compositions that result in the formation of absorbable or non-absorbable hydrogels by appropriate structural design of the hydrogel forming precursors.
It is another object of this invention to provide methods of using the hydrogels of the present invention to reduce post-surgical tissue adhesion, plug lumens, seal tissue, glue tissues together and support or augment tissue either from within the tissue matrix, over the tissue surface, or as an adjunct with another supporting biomaterial.
It is a further object of this invention to provide methods and compositions for controlled and local, regional or systemic drug delivery.
These and other objects of the invention are accomplished by providing compositions forming hydrogels in situ through a combination of physical and chemical crosslinking processes. Preferably, the hydrogel precursor components are all selected to be xe2x80x9csubstantially water soluble,xe2x80x9d having a solubility of at least 1 mg/L in an aqueous solution or in a mix of one or more water miscible solvents compatible for use in a physiological environment.
Physical crosslinking is mediated by one or more natural or synthetic components that act individually or synergistically to stabilize the hydrogel-forming precursor solution at a deposition site for a period of time sufficient for more resilient chemical crosslinks to form. The physical crosslinks may result from complexation, hydrogen bonding, desolvation, Van der Waals interactions, ionic bonding, etc., and may be initiated by mixing two components that are physically separated until combined in situ, or as a consequence of a prevalent condition in the physiological environment, such as temperature, pH, ionic strength, etc.
Chemical crosslinking is mediated by a single or multiple precursor components that form a covalently crosslinked network, thereby providing physical integrity and permanence (or absorbability) to the hydrogel implant. If the hydrogel is to be bioabsorbable, the polymerizable precursor components preferably include at least one linkage susceptible to cleavage, either by hydrolysis, by thermal degradation, or by other physiological processes including enzymatic or cellular activity. The chemical crosslinking may be accomplished by any of a number of mechanisms, including free radical polymerization, condensation polymerization, anionic or cationic polymerization, step growth polymerization, etc.
Methods of using the hydrogels of the present invention as tissue coatings to prevent postsurgical adhesion formation, as tissue augmentation or luminal occlusion aids, as matrices for carrying cells, drugs or other bioactive species, as tissue sealants or adhesives, and as medical device coatings are also provided.